Radio receiver and radio signal processing method

ABSTRACT

A radio receiver comprises a first amplifier to amplify a received radio signal; a quadrature demodulator to demodulate the radio signal amplified by said first amplifier and thereby generate a baseband signal; a second amplifier to amplify the baseband signal; a demodulator to demodulate the baseband signal amplified by said second amplifier; and a gain controller to control timing of a change in a gain of said second amplifier, in case that changing the gain of said first amplifier and the gain of said second amplifier, on the basis of a gain of said first amplifier before and after the change.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of and claims the benefit of priorityunder 35 USC § 120 from U.S. Ser. No. 10/819,288, filed Apr. 7, 2004,claims the benefit of priority under 35 U.S.C. §119 from the priorJapanese Patent Application No. 2003-122316, filed on Apr. 25, 2003, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio receiver and a radio signalprocessing method.

2. Related Background Art

In recent years, radio communication devices have widely spread. As aresult, it is increasingly demanded to reduce the number of componentsand the manufacture cost for a radio circuit and manufacture the radiocircuit as a monolithic IC. In order to cope with this demand, thedirect conversion scheme is adopted for the radio circuits.

FIG. 8 is a block diagram of a conventional receiver adopting the directconversion scheme. An antenna 10 receives an RF (Radio Frequency)signal, and an LNA (Low Noise Amplifier) 20 amplifies this RF signal. Aquadrature demodulator 30 multiplies the amplified RF signal by an LO(Local Oscillator) signal supplied from a local oscillator (notillustrated). As a result, the RF signal is directly converted to abaseband signal. An LPF (Low Pass Filer) 40 conducts waveform shaping onthe baseband signal, and a VGA (Variable Gain Amplifier) 50 amplifiesthis baseband signal. In addition, a demodulator 70 demodulates thisbaseband signal to a digital signal. In this way, the receiver using thedirect conversion scheme converts the RF signal to a digital signal, andthen demodulates it by using digital signal processing.

The baseband signal amplified by the VGA 50 is input not only to thedemodulator 70 but also to a gain controller 60 and a DC offsetcanceller 94. In the gain controller 60, a signal strength detector 80measures the strength of the baseband signal. A gain selector 90 decideswhether to switch the gain of the LNA 20 and the gain of the VGA 50 onthe basis of the measured value of the baseband signal. A gain controlsignal generator 92 outputs a gain control signal to the LNA 20 and theVGA 50 to switch the gain in accordance with the decision made by thegain selector 90. In this way, the gain controller 60 effects feedbackcontrol on the strength of the baseband signal.

The DC (Direct Current) offset canceller 94 removes a DC offsetcomponent from the baseband signal amplified by the VGA 50, and feedsback this to the VGA 50. By the way, problems concerning the DC offsetcomponent are described in “Research development tendency of mixer fordirect conversion receiver(“Mission Impossible ? A Review of Mixers forDirect-Conversion Receivers”)” written by Hiroshi Tanimoto, TheTransactions of the Institute of Electronics, Information, andCommunication Engineers, section C, Vol. J84-C, No. 5, pp. 337-348, May2001.

FIGS. 9A and 9B are graphs showing gains of the LNA 20 and the VGA 50,respectively. FIG. 9C is a graph showing a transient response componentof a DC offset in the output of the VGA 50. The gains of the LNA 20 andthe VGA 50 are simultaneously switched.

When the LNA 20 is switched from a high gain to a low gain at a point t₁in time, the VGA 50 is switched simultaneously from a low gain to a highgain. In some cases, therefore, DC offsets of both the LNA 20 and theVGA 50 overlap eachother, and a very large transient response componentoccurs, resulting in a degraded reception performance. This is becausethe LNA 20 is disposed in a stage in the radio circuit preceding the VGA50, and consequently the transient response component of the DC offsetgenerated in the LNA 20 is amplified by the high gain obtained after thechange in the VGA 50.

SUMMARY OF THE INVENTION

An advantage of an aspect of the present invention is to provide a radioreceiver of direct conversion scheme in which degradation of thereception performance caused by the transient response component of theDC offset is suppressed.

A radio receiver of an embodiment accordance with the instant inventioncomprises a first amplifier to amplify a received radio signal; ademodulation circuit line comprising a quadrature demodulator todemodulate the radio signal amplified by said first amplifier and togenerate a baseband signal, a second amplifier to amplify the basebandsignal, and a demodulator to demodulate the baseband signal amplified bysaid second amplifier; and a gain controller to control timing of achange in a gain of said second amplifier, in case that the gain of saidfirst amplifier and the gain of said second amplifier are changed, onthe basis of a gain of said first amplifier before and after the change.

A radio receiver of another embodiment accordance with the instantinvention comprises a radio receiver comprising: a first amplifier toamplify a received radio signal; a quadrature demodulator to demodulatethe radio signal amplified by said first amplifier and to generate abaseband signal; a second amplifier to amplify, after changing a gain ofsaid first amplifier, the baseband signal with a gain based on a gain ofsaid first amplifier obtained before and after the change; and ademodulator to demodulate the baseband signal amplified by said secondamplifier.

A radio receiver of further embodiment accordance with the instantinvention comprises a first amplifier to amplify a received radiosignal; a quadrature demodulator to demodulate the radio signalamplified by said first amplifier and to generate a baseband signal; asecond amplifier to amplify the baseband signal; a demodulator todemodulate the baseband signal amplified by said second amplifier; and again controller to delay timing of a change in a gain of said secondamplifier as compared with timing of a change in a gain of said firstamplifier, in case that the gain of said first amplifier is changed froma high gain to a low gain and the gain of said second amplifier ischanged from a low gain to a high gain.

A radio signal processing method of an embodiment accordance with theinstant invention comprises: receiving a radio signal; amplifying theradio signal; demodulating the amplified radio signal to a basebandsignal; amplifying the baseband signal; demodulating the amplifiedbaseband signal; determining timing of a change in a gain of said firstamplifier and a gain of said second amplifier, in case that the gain ofsaid first amplifier and the gain of said second amplifier are changed,on the basis of the gain of said first amplifier obtained before andafter the change; and changing the gain of said first amplifier and thegain of said second amplifier in accordance with the determined timing.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an embodiment according to the presentinvention;

FIG. 2 is a block diagram showing a concrete example of a change timingcontroller 196;

FIGS. 3A to 3F are time charts showing gains of the LNA 120 and VGA 150,and signal strength of a baseband signal;

FIGS. 4A to 4H are time charts showing gains of the LNA 120 and VGA 150during phasing and signal strengths of a received signal and a basebandsignal;

FIG. 5 is a flow diagram showing operation of a radio receiver in anembodiment;

FIG. 6 is a flow diagram showing details of operation of a gaincontroller 160 at a step S70;

FIG. 7 is a graph showing a DC offset component induced when the gain ofthe LNA 120 is changed from a high gain to a low gain and the gain ofthe VGA 150 is changed from a low gain to a high gain;

FIG. 8 is a block diagram of a conventional receiver; and

FIGS. 9A to 9C are graphs showing gains of conventional LNA 20 and VGA50.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, embodiments according to the present invention will bedescribed with reference to the drawings. These embodiments do notrestrain the present invention. In a radio receiver of a directconversion scheme according to embodiments of the present invention, thegain of the LNA and the gain of the VGA can be changed respectively atpoints in time that are different from each other. As a result, thetransient response component of the DC offset in the output of the VGAis reduced.

FIG. 1 is a block diagram of a radio receiver 100 according to anembodiment of the present invention. The radio receiver 100 is a radioreceiver using the direct conversion scheme. The direct conversionscheme is a scheme in which an RF signal having a high frequency isconverted to a baseband signal having a low frequency without using anintermediate frequency. The radio receiver 100 includes an antenna 110,an LNA 120, a quadrature demodulator 130, an LPF 140, a VGA 150, a gaincontroller 160, a demodulator 170 and a DC offset canceller 194.

The DC offset canceller 194 is, for example, a circuit formed byconnecting an amplifier having a constant gain and an integrator (lowpass filter) in cascade. Owing to such a configuration, the DC offsetcanceller 194 can remove the DC offset component. The DC offsetcanceller 194 removes a DC offset component contained in the basebandsignal, and then feeds back this baseband signal to the VGA 150. The DCoffset component is induced by a component of an LO signal that leaks tothe antenna 110 and the LNA 120 and undergoes frequency conversion as aninput of the quadrature demodulator 130.

Each of the LNA 120 and the VGA 150 is formed so as to be able to bechanged stepwise in gain. In the present embodiment, the gain of the LNA120 can be changed to two levels, i.e., a high-gain level and a low-gainlevel. The gain of the VGA 150 can be changed to multiple levels betweena high gain and a low gain inclusive thereof.

The gain controller 160 is formed so as to effect feedback control onthe gains of the LNA 120 and the VGA 150 in order to keep a basebandsignal supplied from the VGA 150 at a predetermined signal strength.

The configuration of the gain controller 160 will now be described inmore detail. The gain controller 160 includes a signal strength detector180, a gain selector 190, a change timing controller 196, a gain controlsignal generator 192 and a delay controller 198. The signal strengthdetector 180 detects the signal strength of the baseband signalamplified by the VGA 150. The gain selector 190 conducts selection onthe gain of the LNA 120 and the gain of the VGA 150 so as to keep thesignal strength of the baseband signal detected by the signal strengthdetector 180 at a constant signal strength. The change timing controller196 controls timing at which the gain of the VGA 150 should be changed,on the basis of the gain of the LNA 120 selected by the gain selector190 and the actual gain of the LNA 120 at the current point in time.

The quadrature demodulator 130, the VGA 150 and the demodulator 170 areconnected in series. Hereafter, this is referred to as a demodulationcircuit line. In the present embodiment, two demodulation circuit linesare connected in parallel after the LNA 120, and used for an I-axiscomponent and a Q-axis component of a received signal, respectively. Onegain controller 160 is connected to the two demodulation circuit linesto control the two VGAs 150 in common. For example, the gain controller160 changes gains of the two VGAs 150 by the same period of time afterchanging the gain of the LNA 120. The gain controller 160 changes gainsof the two VGAs 150 by the same quantity. In this way, the gaincontroller 160 controls a plurality of demodulation circuit lines incommon. As a result, the radio receiver 100 can demodulate the I-axiscomponent and the Q-axis component of the received signal in common.

FIG. 2 shows a concrete example of the change timing controller 196. Thechange timing controller 196 includes a gain comparator 201 and a delaycontrol signal generator 203.

The gain comparator 201 compares the actual gain of the LNA 120 at thecurrent point in time with the gain of the LNA 120 selected by the gainselector 190. The gain comparator 201 previously stores a certainthreshold concerning the gain of the LNA 120. The high gain of the LNA120 is a gain larger than the threshold, and the low gain is a gainsmaller than the threshold. The gain comparator 201 compares the gain ofthe LNA 120 before a change with the gain of the LNA 120 after thechange. Then, with the comparison result, the gain comparator 201decides whether the gain of the LNA 120 is changed from the high gain tothe low gain, the gain of the LNA 120 is changed from the low gain tothe high gain, or the gain of the LNA 120 is not changed. Here, the gainof the LNA 120 before the change means the actual gain of the LNA 120 atthe current point in time, and the gain of the LNA 120 after the changemeans the gain of the LNA 120 selected by the gain selector 190.Furthermore, in the present embodiment, changing the gain meansswitching the gain stepwise. By the way, the gain comparator 201 alsopreviously stores a certain threshold concerning the gain of the VGA150. The high gain of the VGA 150 is a gain larger than the threshold,and the low gain of the VGA 150 is a gain smaller than the threshold.

The delay control signal generator 203 generates a delay control signalthat indicates a delay time used to delay the change in gain of the VGA150. In the case where the gain of the LNA 120 is changed from the highgain to the low gain, the delay control signal generator 203 generates adelay control signal when the gain of the VGA 150 is changed from thelow gain to the high gain. This delay control signal is output to thedelay controller 198. On the other hand, in the case where the gain ofthe LNA 120 is changed from the low gain to the high gain or the gain ofthe LNA 120 is not changed, the delay control signal generator 203 doesnot generate the delay control signal when the gain of the VGA 150 ischanged.

The gain control signal generator 192 is supplied with the gain selectedin the gain selector 190 via the change timing controller 196. The gaincontrol signal generator 192 outputs a gain control signal to the LNA120 and the delay controller 198 on the basis of the gains of the LNA120 and the VGA 150 selected by the gain selector 190. The gain controlsignal is a signal indicating the gains respectively of the LNA 120 andthe VGA 150 selected by the gain selector 190.

The delay controller 198 outputs the gain control signal to the VGA 150,after a predetermined delay time has elapsed since a point in time atwhich the delay controller 198 receives the gain control signal, inaccordance with the delay control signal. Since the gain control signalis transmitted directly to the LNA 120, the gain of the VGA 150 ischanged with a delay to the gain of the LNA 120. On the other hand, inthe case where the delay control signal is not output from the delaycontrol signal generator 203, the delay controller 198 outputs the gaincontrol signal to the VGA 150 without delaying the gain control signal.

In this way, the gain controller 160 is formed so as to control thetiming at which the gain of the VGA 150 is changed on the basis of thegain of the LNA 120 before and after the change.

When the gain of the LNA 120 is changed, it is evident in the presentembodiment that the gain change of the VGA 150 is brought about.Therefore, the timing at which the gain of the VGA 150 is changed iscontrolled on the basis of the gain of the LNA 120 before and after thechange.

In the case where it is not evident that the gain change of the VGA 150is brought about when the gain of the LNA 120 is changed, however, thetiming at which the gain of the VGA 150 is changed may be controlled onthe basis of the gains of the LNA 120 and the VGA 150 before and afterthe change.

FIGS. 3A to 3F are time charts showing gains of the LNA 120 and the VGA150, and the signal strength of the baseband signal. With reference toFIGS. 3A to 3F, operation of the LNA 120 and the VGA 150 will now bedescribed in further detail.

First, the gain of the LNA 120 is changed from the low gain to the highgain as shown in FIG. 3A, and the gain of the VGA 150 is changed fromthe high gain to the low gain as shown in FIG. 3B. The gains of the LNA120 and the VGA 150 are changed at a point in time t₂₀. A transientresponse characteristic of the DC offset induced at this time isrelatively small as shown in FIG. 3C.

Subsequently, the gain of the LNA 120 is changed from the high gain tothe low gain as shown in FIG. 3D, and the gain of the VGA 150 is changedfrom the low gain to the high gain as shown in FIG. 3E. The gain of theLNA 120 is changed at a point in time t₂₁. If at this time the gain ofthe VGA 150 is changed simultaneously with the change in the gain of theLNA 120 as represented by a broken line in FIG. 3E, a large transientresponse component of the DC offset is induced as represented by abroken line in FIG. 3F.

In the present embodiment, therefore, the gain of the VGA 150 is changedfrom the low gain to the high gain with a delay time Td after the changein the gain of the LNA 120 as represented by a solid line in FIG. 3E.The delay time Td is represented by Td=t₃₁−t₂₁. As a result, thetransient response component of the DC offset induced at the point intime t₂₁ becomes smaller than the transient response component inducedin the conventional technique. The delay time Td is larger than 0, andsmaller than a repetition period (Δt shown in FIG. 9) of the change ingains of the LNA 120 and the VGA 150.

In this way, the gain of the VGA 150 is changed with a delay to thechange in the gain of the LNA 120, in the present embodiment. Therefore,the transient response component of the DC offset can be reduced.

Furthermore, in FIG. 3F, an area S_(B) of a region surrounded by astraight line L and the solid line is obviously smaller than an areaS_(A) of a region surrounded by the straight line L and the broken line.In the present embodiment, therefore, the DC offset component per unittime is smaller than that in the conventional technique. Since, asdescribed above, the error rate in the reception characteristics isproportionate to an accumulation value of an area S per unit time, thepresent embodiment has a smaller error rate in the reception performancethan that of the conventional technique. In the present embodiment,therefore, the reception performance becomes better than that in theconventional technique.

Although in the present embodiment the gain of the LNA 120 can bechanged to two levels, it is also permissible that the gain of the LNAcan be changed to three or more levels.

FIGS. 4A to 4H are time charts showing gains of the LNA 120 and VGA 150in the case where the received electric field strength (so called “RSSI(received signal strength indicator))changes monotonously, and timecharts showing signal strengths of the received signal and signalstrengths of the baseband signal. A variant for the embodiment shown inFIGS. 3A to 3F will now be described with reference to FIGS. 4A to 4H.

First, the case where the signal strength of the received signalsupplied from the antenna 110 falls between a point in time t₁₀ and apoint in time t₃₀ as shown in FIG. 4A will now be described. The gain ofthe VGA 150 gradually rises stepwise from the point in time t₁₀ as shownin FIG. 4C under the feedback control of the gain controller 160. As aresult, the amplification factor for the received signal rises even ifthe signal strength of the received signal falls. Therefore, the signalstrength of the baseband signal is kept constant as shown in FIG. 4D.

However, there is an upper limit in the gain of the VGA 150. If the gainof the VGA 150 arrives at a vicinity of its upper limit at the point intime t₂₀, therefore, the gain of the LNA 120 is changed from the lowgain to the high gain as shown in FIG. 4B, and the gain of the VGA 150is changed from the high gain to the low gain as shown in FIG. 4C. Thegain change widths of the LNA 120 and the VGA 150 are nearly equal toeach other. As a result, the fall in the gain of the VGA 150 can becompensated by the increase in the gain of the LNA 120. The transientresponse characteristic of the DC offset induced at this time isrelatively small as shown in FIG. 4D. In the present embodiment, thegain of the VGA 150 is changed in a larger number of steps as comparedwith the gain of the LNA 120 as shown in FIGS. 4B, 4C, 4F and 4G. Evenif the received signal strength changes linearly as shown in FIG. 4A,therefore, the signal strength of the baseband signal can be keptconstant in the present embodiment.

Between the points in time t₂₀ and t₃₀, the signal strength of thereceived signal further continues to fall. In such a case, the signalstrength of the baseband signal can be kept constant by making the gainof the VGA 150 further rise stepwise. In FIG. 4D and FIG. 4H describedlater, the transient response component of the DC offset caused by thestepwise gain switching of the VGA 150 is omitted, because it is small.

Subsequently, the case where the signal strength of the received signalrises between a point in time t₁₁ and a point in time t₃₁ as shown inFIG. 4E will now be described. The gain of the VGA 150 gradually fallsstepwise from the point in time t₁₁ as shown in FIG. 4G under thefeedback control of the gain controller 160. As a result, theamplification factor for the received signal falls even if the signalstrength of the received signal rises. Therefore, the signal strength ofthe baseband signal is kept constant as shown in FIG. 4H.

However, there is a lower limit in the gain of the VGA 150. If the gainof the VGA 150 arrives at a vicinity of its lower limit at the point intime t₂₁, therefore, the gain of the LNA 120 is changed from the highgain to the low gain as shown in FIG. 4F. If at this time the gain ofthe VGA 150 is changed simultaneously with the change in the gain of theLNA 120, a large transient response component of the DC offset occurs atthe point in time t₂₁ as represented by a broken line in FIG. 4H.

In the present variant, the gain of the VGA 150 is changed from the lowgain to the high gain with a delay time Td after the change in the gainof the LNA 120 as represented by a solid line in FIG. 4G. At this time,the gain change widths of the LNA 120 and the VGA 150 are nearly equalto each other. The delay time Td is represented by Td=t₃₁−t₂₁. As aresult, a transient response component of the DC offset induced at thepoint in time t₂₁ becomes smaller as compared with the transientresponse component in the conventional technique as shown in FIG. 4H.The delay time Td is a value that is larger than 0 and that is smallerthan a repetition period (At shown in FIG. 9) of the change in gains ofthe LNA 120 and the VGA 150.

Between the points in time t₂₁ and t₃₁, the signal strength of thereceived signal further continues to rise. In this case, the signalstrength of the baseband signal can be kept constant by making the gainof the VGA 150 fall gradually.

Thus, in the present variant, the gain of the VGA 150 is changed with adelay to the change in the gain of the LNA 120, and consequently effectssimilar to those of the embodiment shown in FIGS. 3A to 3F can beobtained.

In the conventional technique, the gain of the VGA 150 is changedbetween the points in time t₂₁ and t₃₁ as represented by a broken linein FIG. 4G. This means that the gain of the VGA 150 is being controlledwhen the transient response component of the DC offset is occurring.Therefore, the gain of the VGA 150 is changed largely at the point intime t₂₁.

On the other hand, in the present variant, the gain of the VGA 150 isnot changed during the delay time Td, i.e., between the points in timet₂₁ and t₃₁. As a result, the width of the gain of the VGA 150 changedat the point in time t₃₁ in the present variant is smaller than that atthe point in time t₂₁ in the conventional technique. Therefore, thetransient response component of the DC offset induced in the presentvariant is relatively small. Furthermore, according to the embodimentshown in FIGS. 3A to 3F, the width of the gain of the VGA 150 changed atthe point in time t₃₁ is equal to that changed at the point in time t₂₁in the conventional technique. Therefore, a peak P₂ Of the transientresponse component of the DC offset induced at the point in time t₃₁ inthe present variant becomes further smaller than a peak P₁ of thetransient response component induced at the point in time t₃₁ in theembodiment shown in FIGS. 3A to 3F.

In the present variant, the gain of the VGA 150 is changed singlywithout changing the gain of the LNA 120 in some cases. In this case,however, the transient response component of the DC offset is relativelysmall similarly as in FIG. 4D, and consequently no problems are posed.

FIG. 5 is a flow diagram showing operation of the radio receiver 100 inthe embodiment. An RF signal is received by the antenna 110 (S10). Thereceived signal is amplified by the LNA 120 (S20). The quadraturedemodulator 130 converts the RF signal having a high frequency to thebaseband signal (S30). The baseband signal is subjected to waveformshaping in the LPF 140 (S40), and amplified in the VGA 150 (S50). The DCoffset canceller 194 removes the DC offset component from this basebandsignal (S60). The gain controller 160 is supplied with the basebandsignal output from the VGA 150, and the gain controller 160 effectsfeedback control on the LNA 120 and the VGA 150 (S70). In addition, thedemodulator 170 demodulates the baseband signal to the digital signal(S80).

FIG. 6 is a flow diagram showing details of operation conducted by thegain controller 160 at the step S70. If the baseband signal is input tothe gain controller 160, the signal strength detector 180 detects thesignal strength of the baseband signal (S70-1).

Subsequently, the gain selector 190 selects gains of the LNA 120 and theVGA 150 so as to keep the signal strength of the baseband signalconstant (S70-3). Subsequently, the change timing controller 196compares the actual gain state of the LNA 120 at the current point intime with the selected gain of the LNA 120 (S70-5). As a result of thiscomparison, it is determined whether the gain of the LNA 120 passesthrough a threshold previously stored in the change timing controller196 before and after a change (S70-6).

If the gain of the LNA 120 passes through this threshold before andafter the change, the change timing controller 196 furthermore judgesthe gain state of the LNA 120 at the current point in time (S70-7).Judgment on the gain state of the LNA 120 can be conducted bydetermining whether the gain of the LNA 120 is higher than thisthreshold (S70-8). If the gain of the LNA 120 at the current point intime is the high gain as a result of this decision, the change timingcontroller 196 outputs the delay control signal to the delay controller198 (S70-9).

Subsequently, the gain control signal generator 192 outputs the gaincontrol signal to the LNA 120 and the delay controller 198 on the basisof the gains of the LNA 120 and the VGA 150 selected by the gainselector 190 (S70-11). The delay controller 198 is supplied with thegain control signal and the delay control signal, and the delaycontroller 198 delays the gain control signal and outputs the delayedgain control signal to the VGA 150. As a result, the gain of the VGA 150is changed with a delay to the change in the gain of the LNA 120(S70-13).

If the gain of the LNA 120 at the current point in time is the low gainat the step S70-8, the change timing controller 196 does not output thedelay control signal. Since the delay control signal is not output, thedelay controller 198 outputs the gain control signal to the VGA 150without delaying it. As a result, the gain of the VGA 150 is changedsimultaneously with a change in the gain of the LNA 120 (S70-15).

If the gain of the LNA 120 does not pass through the threshold beforeand after the change at the step S70-6, the delay control signal is notoutput. The change timing controller 196 further makes a decisionwhether to change the gain of the VGA 150 (S70-17). In this decision,the gain comparator 201 previously stores a threshold located betweenthe low gain and the high gain of the VGA 150, and judges a gain higherthan the threshold to be the high gain and judges gain lower than thethreshold to be the low gain. If the gain of the VGA 150 passes throughthis threshold before and after the change, the change timing controller196 decides to change the gain of the VGA 150 (S70-18).

If the gain of the VGA 150 is decided to be changed, the gain controlsignal generator 192 outputs the gain control signal to the VGA 150 viathe delay controller 198 to change the gain of the VGA 150. Since thedelay control signal is not issued from the change timing controller196, the delay controller 198 passes the gain control signal to the VGA150 without delaying it. As a result, the gain of the VGA 150 is changed(S70-19).

If the gain of the VGA 150 is decided not to be changed, the delaycontroller 198 does not change the gain of VGA 150.

FIG. 7 is a graph showing a DC offset component actually measured whenthe gain of the LNA 120 is changed from the high gain to the low gainand the gain of the VGA 150 is changed from the low gain to the highgain. FIG. 7 corresponds to the graph shown in FIG. 4H in which thesignal strength of the baseband signal has been obtained by actuallymeasuring it. A curve A shows a DC offset component measured when thegain of the LNA 120 and the gain of the VGA 150 are changedsimultaneously in the same way as the conventional technique. A curve Bshows a DC offset component measured when the gain of the VGA 150 ischanged with a delay to a change in the gain of the LNA 120 according tothe embodiment. These graphs are data showing actual measured resultsobtained when an RF signal from a signal generator is input to the LNA120 and a baseband signal output from the VGA 150 is observed on adigital oscilloscope.

In the curve A, the transient response component of the DC offset outputfrom the VGA 150 is approximately 70 mV maximum. On the other hand, inthe curve B, the transient response component of the DC offset outputfrom the VGA 150 is approximately 30 mV maximum. Therefore, thetransient response component in the curve B is obviously lower than thetransient response component in the curve A.

In the case where a threshold is provided for the DC offset component,the probability in the embodiment that the DC offset component exceedsthe threshold becomes lower as compared with the conventional technique.

An area S_(B) of a region surrounded by a curve B and a broken line isobviously smaller than an area S_(A) of a region surrounded by a curve Aand the broken line. In the embodiment, therefore, the DC offsetcomponent per unit time is smaller as compared with the conventionaltechnique. Since the error rate in the reception characteristic isproportionate to the accumulation value of the area S per unit time asdescribed earlier, the error rate in the reception characteristic in theembodiment is smaller as compared with the conventional technique. As aresult, the embodiment becomes better in reception performance than theconventional technique.

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand example embodiments will be considered as exemplary only, with atrue scope and spirit of the invention being indicated by the following.

1-20. (canceled)
 21. A radio signal processing method in a radioreceiver, the radio receiver comprising a first amplifier, a secondamplifier and a gain controller controlling gains of the first and thesecond amplifier, the method comprising: receiving a radio signal;amplifying a received radio signal in the first amplifier; demodulatingthe amplified radio signal to a baseband signal; amplifying the basebandsignal in the second amplifier; demodulating the amplified basebandsignal; controlling timing, in the gain controller, of a change in again of the first amplifier, in case that the gain of the firstamplifier and the gain of the second amplifier are changed, on the basisof a gain of the first amplifier before and after the change; detectingstrength of the baseband signal; selecting the gain of the firstamplifier and the gain of the second amplifier at the gain controller onthe basis of the strength of the baseband signal; comparing actual gainsof the first amplifier and the second amplifier with the selected gainof the first amplifier and the selected gain of the second amplifier;determining timing of a change in the gain of the second amplifier onthe basis of a result of the comparison; generating a control signal tochange the gain of the first amplifier and the gain of the secondamplifier; and transmitting the control signal to the second amplifierin accordance with the determined timing.
 22. The method according toclaim 21, wherein in a case that said gain controller changes the gainof said first amplifier from a high gain to a low gain and changes thegain of said second amplifier from a low gain to a high gain, the gaincontroller transmits the control signal to said second amplifier a delayafter transmission of the control signal to said first amplifier. 23.The radio receiver according to claim 21, wherein a delay time isshorter than a repetition period of the changes in the gain of saidfirst amplifier and the gain of said second amplifier, said delay timebeing a period from the change in the gain of said first amplifier untilthe change in the gain of said second amplifier.
 24. A method ofprocessing a radio signal in a radio receiver, the radio receivercomprising a first amplifier, a second amplifier and a gain controllercontrolling a gain of the first and the second amplifier, the methodcomprising: receiving a radio signal; amplifying a received radio signalin the first amplifier; demodulating the amplified radio signal to abaseband signal; amplifying the baseband signal in the second amplifier,after changing a gain of said first amplifier, with a gain based on again of said first amplifier obtained before and after the change; anddemodulating the amplified baseband signal, wherein in a case that thegain of said first amplifier is changed from a high gain to a low gainand the gain of said second amplifier is changed from a low gain to ahigh gain, timing of the change in the gain of said second amplifier isdelayed as compared with timing of the change in the gain of said firstamplifier.
 25. The method according to claim 24, wherein a delay time isshorter than a repetition period of the changes in the gain of saidfirst amplifier and the gain of said second amplifier, said delay timebeing a period from the change in the gain of said first amplifier untilthe change in the gain of said second amplifier.
 26. A radio signalprocessing method comprising: receiving a radio signal; amplifying theradio signal; demodulating the amplified radio signal to a basebandsignal; amplifying the baseband signal; demodulating the amplifiedbaseband signal; determining timing of a change in a gain of the radiosignal amplification and a gain of the baseband signal amplification, incase that the gain in the radio signal amplification and the gain in thebaseband signal amplification are changed, on the basis of the gain inthe radio signal amplification obtained before and after the change; andchanging the gain in the radio signal amplification and the gain in thebaseband signal amplification in accordance with the determined timing,wherein in a case where the gain in the radio signal amplification ischanged from a high gain to a low gain and the gain in the basebandsignal amplification is changed from a low gain to a high gain, in acase that the timing of the change in the gain is determined, the gainin the baseband signal amplification is changed with a delay to thechange in the gain in the radio signal amplification.